Mugurel,
Forget that plan that you used, nothing is scale enough on it, everything is not correct.
It is the best idea and easier way to start a new fuselage than to modify the one you built via that semi-scale plan.
Get Eduard MiG-21MF 1/48 kit, slice it to get cross sections, enlarge them and work from there ..
Use a lot of the real MIG photos from internet and make corrections accordingly to them.
Sounds easy, but it would take some effort, however that way the model you are building would be much more scale.
Taking into the account a lot of the hard work, time, money and effort, it would be a well worth it to make the MIG more correct in scale ..
Best regards,
Kris